Systems which handle material, such as manufacturing facilities, often employ automated material handling systems to move various materials from one location to another location within the system. The materials may include raw materials, finish product materials, or any materials inbetween. One example of such a manufacturing facility is a semiconductor fabrication facility.
A typically semiconductor fabrication facility includes multiple factory tools interconnected by one or more paths, such as conveyor belts. Factory tools may include storage tools, such as stockers or WIP racks, and processing tools, for example. Each of the processing tools is used to carry out a particular process step on the semiconductor wafers. Exemplary processing tools include photolithography tools, chemical mechanical polishing tools and chemical vapor deposition tools, for example. The wafers are typically stored in and moved about through fabrication facility using material carriers, such as cassettes. The wafers are typically divided into identifiable lots each of which undergo a particular processing sequence. Each of the cassettes or material carriers also includes an identification tag for identification.
A typical semiconductor fabrication facility usually includes a factory system for generating wafer lot move requests and an automated material handling system (AMHS) for handling the move requests. The factory system typically stores a processing sequence for each wafer lot and sends requests to move the wafer lot to the AMHS based on the position of the wafer lot within its fabrication sequence. The AMHS typically includes a controller associated with each of the storage tools, for example. Typically the move requests from the factory system identify a particular wafer lot and a destination tool for the wafer lot. The move request is forwarded to the controller associated with the present factory tool of the wafer lot. This controller then handles the movement of the wafer lot to its destination tool. This typically includes reserving a place in the destination factory tool for a cassette carrying the wafer lot and placing the cassette carrying the wafer lot onto one or more paths for transportation of the wafer lot to the destination tool.
As semiconductor wafers increase in complexity, the manufacture of such materials also increases. For example, different types of fabrication sequences may be employed and thus a higher number of wafer lots may be processed in a factory and a larger number of cassettes may be used in the factory. The additional wafer lots and cassettes increase the complexity of the fabrication process and place additional burdens on the AMHS. This increases the propensity of the wafers and wafer carriers back up or “log jam”. These log jams severely reduce the throughput of semiconductor wafers. As less throughput is of high concern for manufacturers, method and systems which reduce log jams are in continual need.